Friday, 27 March 2015

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

The LHC did not see a circulating beam this week due to a short circuit in one of the magnets, likely due to debris in a diode box. Delay could be days or months. Read the CERN press release here, and there's some more information at Nature News. [Edit: more from CERN here.]

The ATLAS/CMS combined Higgs mass measurement is now up on the arXiv (~6000 authors!).

There has been a bit more talk on the LHCb $B\to K^*\mu\mu$ excess which was updated from 1/fb to 3/fb of data last week. The Conf Note is now up. There's a short blog post on interpretation from David Straub, and the Straub/Altmannshofer Proceedings paper has quite a nice summary. See also the Resonaances post and the previous-two posts also there.

Shown below is the old result (blue) and new result (black) against theory (orange) for the angular observable of interest.

The point is that the 4--8 GeV^2 bins each deviate from the theory prediction, and a naive combination suggests a 3.7σ tension. This happens to be the same tension seen in the analysis with 1/fb, which suggests that the new data have still fluctuated up, but not quite as much. The effect has not gone away. But I am with Tommaso... I would like to know what is the probability of seeing such a deviation or larger in any of the angular observables they looked at, assuming the null. Probably this is difficult to do; likely there are significant correlations between bins of different observables(?)... still, it would be nice to know a number for the global significance as well.

The question now is whether we are seeing new physics or underestimated theory uncertainty. The data are not suggesting any problem with form factors, but an unexpectedly large charm-loop contribution near $J/\psi$ is a possible explanation. If it is new physics, then the discrepancy can be explained by a single operator$$O_9=(\bar{s}\gamma_\mu P_L b)(\bar{l}\gamma^\mu l)$$quantified by a parameter $C_9^{NP}$. This could also go some way to explaining the 2.6σ deviation from lepton-flavour universality (quantified by $R_K$) also measured by LHCb. According to the Straub/Altmannshofer Proceedings there are sensible ways to proceed if we want to discover the culprit. Here are a few of them:

1. Keep looking at q^2 dependence of $C_9$; the new physics effect should be q^2 independent.

2. Measure $B\to K^*\mu\mu$ and $B\to K^*ee$ branching ratios and angular observables. If the same new physics is responsible for this and $R_K$ then you might see "spectacular" deviations.

3. Search for lepton flavour violations in $B\to K^*\mu e$.

Certainly these are all measurements to look forward to...

If you want more insight into what's going on at Moriond, their twitter stream is excellent.

There's another excess that you might see making the rounds soon. They're everywhere! This week it is >2σ in a search for WH resonances in $l\nu b\bar{b}$.

Of interest are the three events at ~1800 GeV. It is only seen in the electron channel, so if it is new physics, it is not probably not a WH resonance. It is almost not worth mentioning, but CMS also saw an excess in their right-handed W search in $lljj$. There they saw a >2σ discrepancy in the electron channel at around the same mass scale.

I have not had time to read about these in detail, but if they can be linked, they will be linked. Watch the arXiv...

Higgs gluon-fusion production cross-section has been computed at N3LO (and Moriond talk here [pdf]), "the first ever complete computation of a cross-section at N3LO at a hadron collider."

Hooper and Linden have weighed in on the Reticulum II gamma-ray excess (seen in one of the newly discovered DES dwarf satellites). From the conclusion: "In order for this excess to be compatible with the lack of significant gamma-ray detections from other dwarf galaxies (most importantly, Segue 1 and Ursa Major II), Reticulum II must contain a high density of dark matter... A measurement of Reticulum II’s J-factor that is much smaller than this value would place serious doubt as to any dark matter interpretation of its excess."

There was lot of hype relating to a lazy LHC article in the media this week. On the plus side, Backreaction has an excellent rebuke (on the article and the paper that spawned it), and there you can also learn a bit about rainbow gravity.

A paper published in Science used the Chandra and Hubble Space Telescopes to observe 72 galaxy cluster collisions and subsequently constrain dark matter self-interaction. This number now supersedes the constraint from the bullet cluster. You can read the press release or watch the astrophysicists involved (along with others) discussing it on YouTube [1 hour].

A paper appeared on the arXiv today, signed by a number of physicists from many different institutions, commenting that the Nature paper on inner galaxy dark matter appearing a month or so back was not even wrong: "Considerable confusion may stem from the use of the term ‘inner’. The Sun’s orbit encompasses roughly 90% of the stellar mass. By this standard, we live in the outskirts of the Galaxy. That some DM is needed interior to the Solar circle is neither surprising nor new."

In video/audio media:

I only discovered the Colliding Particles series of shorts today, following a team of physicists involved in the research at the LHC. You can watch them here.

Friday, 20 March 2015

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

Long Shutdown (LS1) is over, and we should have the first fully circulating beam next week!

CMS Technical Coordination: "LS1 is over. Thks to those who made it a success. CMS is ready for beam. We look forward to Run 2." #RestartLHC
— CMS Experiment CERN (@CMSexperiment) March 19, 2015

You can track LHC and CMS status here and even view live events. CMS are currently doing their cosmic run with the magnet only turned up a couple of days ago; I managed to catch a nice one bending in the B field:

The ATLAS/CMS preliminary Higgs mass combination was presented for the first time at Moriond (talk here [pdf]). The result is $$m_H=125.09\pm0.24\, [\pm0.21\text{ (stat.)}\pm0.11\text{ (syst.)] GeV}$$

ATLAS has submitted their results on $t\bar{t}H$ production with $H\to b\bar{b}$ and released a Conf Note on $H\to WW,\tau\tau,ZZ$. Both see a small excess over the SM (of some interest only because CMS has $\mu_{ttH}$ at $>2\sigma$ above SM). I assume that those results enter into the following plot also presented in the Moriond talk(?). At least it is something to keep half an eye on in Run 2, while it likely goes away...

There's a rumour (via Jester) that the LHCb $B\to K^*\mu\mu$ analysis for the full 3/fb of data is due out soon, and it confirms the anomaly already observed in the previous analysis. If it is today then that would line up with the heavy flavour day at Moriond. See these-two Resonaances posts for an easily digestible recap, or dig deeper with any of these...

NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has observed two unexpected phenomena in the Martian atmosphere: an unexplained high-altitude dust cloud and aurora that reaches deep into the Martian atmosphere. No mention in the NASA release or the Nature News, but I wonder if perhaps they are seeing again the plumes that hit the news a few weeks back?

Let's end on a shot from space of the aurora from the St. Patrick's Day Solar Storm. Next week I'm sure we'll have some equally impressive shots of the total solar eclipse happening today!

Friday, 13 March 2015

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

ATLAS released a preprint yesterday (submitted to EPJC), Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum..., that is interesting for two reasons.

2. They call CMS's excess, and then they raise, with a 3.0σ excess in a different signal region (SR). So let's talk about that...

The search is for an on-Z opposite-sign same-flavour (OSSF) lepton pair + jets + MET. They are motivated by a gravitino LSP SUSY scenario with pair-produced gluinos which decay via $\tilde g\to qq\tilde\chi_1^0, \tilde\chi_1^0\to Z\tilde G$ (though it seems to me like something as simple as a vector-like quark could also work). Anyway, after typical preselection and requiring two OSSF leptons (if more than two are present they take the leading leptons), they define the on-Z signal region as: $$81< m_{l^+l^-}/\text{GeV}<101, \\ n_{jets}\ge2, \\ E_T^{miss}>225\text{ GeV,} \\ H_T>600\text{ GeV,} \\ \Delta\phi(jet_{12},E_T^{miss}>0.4,$$where $H_T$ is the scalar sum of the jet and lepton $p_T$ in the event, and the $\Delta\phi$ cut is designed to reject background from mismeasured jets faking large $E_T^{miss}$. And backgrounds are tough... $Z/\gamma^*+jets$ with mismeasured jets producing difficult-to-model instrumental $E_T^{miss}$ is potentially worrisome, but it is made negligible by the $\Delta\phi$ cut. Flavour-symmetricbackgrounds (with a truth-level flavour ratio $ee:\mu\mu:e\mu$ of 1:1:2) from $t\bar{t}$, $WW$, single top, and $Z\to\tau\tau$ are dominant; they are estimated with a data-driven method using opposite-flavour data as a control region. Fake leptons are estimated from data. Diboson, $t\bar{t}V$, $t\bar{t}VV$, and $t+Z$ are estimated from MC, making sure not to double count the flavour-symmetric component.

The expected and observed number of events as a function of invariant mass in the dielectron and dimuon channels are shown below:

For the sum of both channels the expected background is 10.6±3.2 with 29 events observed, which ends up corresponding to a 3.0σ excess.

Now, CMS did a similar search in the on-Z SR in their paper and didn't see anything. So are the results consistent? It's possible. The CMS SR wasn't quite as tight as the one employed by ATLAS. After similar preselection, for an on-Z signal region defined as $$81< m_{l^+l^-}/\text{GeV}<101, \\ n_{jets}\ge2, \\ E_T^{miss}>200\text{ GeV,}$$CMS have an expected background of $\approx$ 87.3±12.1 with 72 events observed. So who knows, maybe if CMS demanded $H_T>600$ GeV they would see something too, or maybe not...

The biggest news of the week comes from Tuesday's astro-ph listings. This is not my area, so I can't comment intelligently, but anyone can read an abstract and look at Figures, so I will just sum up here for completeness and convenience (click the figures to make them larger)...

1. Fermi-LAT released their Pass 8 constraints on dark matter annihilation (already largely known from preliminary results). They rule out dark matter masses $\lesssim 100$ GeV for a thermal relic annihilating to $b\bar{b}$ or $\tau\tau$. Those results are cutting into the best fit regions for the galactic centre excess.

2. The Dark Energy Survey (DES) Collaboration has located eight new dwarf satellite galaxy candidates (of the Milky Way and/or Magellanic Clouds), and an independent Cambridge group has located nine using the publicly released DES deep photometry data. You can read the press release here.

3. The new satellite candidates are prime spots to look for dark matter annihilation... so Fermi-LAT went and did it already! Assuming that the new candidates are dwarf spheroidals, they set a limit on the annihilation cross-section that rivals their Pass 8 results with known dwarfs above.

4. But the story isn't over yet, because an independent group (which includes the Cambridge group that found nine candidates) has reported a gamma-ray excess, consistent with DM annihilation, in one of the new dwarf candidates. [Edit: The candidate is Reticulum II or DES J0335.6−5403, the green line in the above Fermi-LAT plot, which appears by eye to be the only line of all the candidates to have a weakened limit in the 10−few×100 GeV DM mass region, the region that would produce the excess.]

And the dark matter annihilation saga continues...

Protons bunches half-circled the LHC beam pipe last weekend for the first time since the long shutdown began! Injector tests sent bunches from the SPS into the LHC ring and through ALICE and LHCb on their way to beam dumps. Both ALICE and LHCb recorded splash events when the beam was made to collide with a target.

You can play with the LHCb event here. First fully circulating beam is expected at the end of the month.

PRL has published the Planck/BICEP2/Keck joint analysis, along with a Viewpoint article which tells some of the story -- we are reminded of the following: "... alternative models may be detectable with the next generation of experiments, some of which claim a sensitivity to r as small as 0.01. The competition is fierce, with at least six funded ground-based experiments underway (including the third version of BICEP), several balloon-borne experiments, and a number of proposed space missions."

There's a nice feature at ScienceNews about the AMS experiment, the positron excess, and Samuel Ting; on the (unreleased) preliminary antiproton data he remarks: "intriguing".

Published in Nature yesterday, the Cassini orbiter has detected tiny rock grains emitted from the plumes of the Saturnian moon Enceladus, hinting at a subsurface ocean. You can read the articles at NASA, ESA, or Scientific American. Meanwhile a team using Hubble have used observations of aurora to indirectly suggest that there is a subsurface ocean on Ganymede, Jupiter's largest moon. Nice to see that there are complementary ways to measure these things.

Today Rosetta is trying to listen for a signal from the Philae lander on Comet 67P/Churyumov-Gerasimenko. ESA released a cartoon video about it a few days ago [3 minutes]. The Lander Project Manager says, "It will probably still be too cold for the lander to wake up, but it is worth trying."

Dawn went into orbit around Ceres successfully last Friday. Science begins in late April.

Friday, 6 March 2015

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

CMS has submitted their paper on the search for LFV Higgs decays. This analysis supersedes the PAS from July (which has 20 citations). They see an excess of 2.4σ, which I wouldn't make much of, but it is worth keeping track of these "hints" because they motivate papers you see popping up on hep-ph. The following is a quick summary...

They search in the $\mu\tau$ channel, which is experimentally the best channel in which you might hope to see something after taking into account bounds from LFV transitions. The best fit branching ratio for the excess corresponds to $Br(H\to \mu\tau)=0.84\%$, an order of magnitude below bounds from $\tau\to\mu\gamma$. The dominant backgrounds for the search are $Z\to\tau\tau$, $W+jets$, QCD multijets, and $t\bar{t}$. The latter three are certainlysome of the nastiest backgrounds around at the LHC. $W+jets$ in particular is responsible for a number of historical anomalies (see Tomasso's blog or his talk). Happily they can be estimated with the help of data in background-enriched regions, which is nice, but shape uncertainties must then be carefully considered as systematics. Anyway, let us move on to the results...

The left and right columns correspond to the leptonic and hadronic $\tau$ decay channels respectively, and each row corresponds to an increasing number of jets (the last row is enhanced with VBF events). The variable they plot against is the collinear mass $M_{col}$, an estimator for the Higgs mass (in the signal events) constructed by assuming the neutrino (coming from the $\tau$ decay) momentum is equal to the projection of the missing momentum vector onto the direction of the $\tau$ decay products. The distributions driving the excess are $\mu\tau_e\;0$-jet, $\mu\tau_e\;1$-jet, and $\mu\tau_h\;2$-jet. That can be seen by eye above and in the branching fits themselves:

One would assume (or hope?) that the ATLAS analysis is underway, and certainly it will be interesting to find out what they see.

ATLAS has bounded the Higgs width at < 22.7 MeV at 95% CL (SM is 4.1 MeV) using the off-shell $gg\to VV$ Higgs boson signal, with assumptions (see below)! This supersedes their Conf Note from July and adds the WW channel to the analysis. The idea of doing this measurement (as far as I know) is based on the paper from Caola/Melnikov (1307). CMS managed to do the analysis and present it as a prepublication within 6 months of that paper (a phenomenal effort), rushed out for Moriond [pdf], and later published (in September) a bound of < 22 MeV at 95% CL [it should be noted that in the present ATLAS analysis the CLs method has been used which weakens their upper bound in the presence of the observed downward fluctuation of the low-statistics background; CMS used the regular $-2\ln L<4$ method and if ATLAS had done the same, judging by their Figure 12, their bound would rather be < 16.4 MeV]. The CMS result was presented as having "mild model-dependence," but not long after the bound was shown to be invalid for general new physics scenarios without some specific assumptions (see e.g. Englert/Spannowsky 1405). ATLAS appear to have specified clearly the assumptions going into their analysis...

The ratio of the off-shell to on-shell g-g fusion signal strength is $$\frac{\mu_{\text{off-shell}}(\hat{s})}{\mu_{\text{on-shell}}}=\frac{\kappa^2_{g,\text{off-shell}}(\hat{s})\kappa^2_{V,\text{off-shell}}(\hat{s})}{\kappa^2_{g,\text{on-shell}}\kappa^2_{V,\text{on-shell}}}\frac{\Gamma_H}{\Gamma_H^{SM}},$$where the $\kappa$ are coupling scale factors of the Higgs to $gg$ and $VV$. The approximation CMS made was that the ratio of $\kappa$'s was equal to unity. ATLAS sets their limit under the assumption $$\kappa_{g,\text{on-shell}}^2\kappa_{V,\text{on-shell}}^2\le \kappa_{g,\text{off-shell}}^2\kappa_{V,\text{off-shell}}^2 .$$They also assume that none of any new physics which might alter the Higgs width changes substantially the background (of particular interest in this case since the off-shell signal and continuum background destructively interfere). The higher-order QCD corrections for the continuum background are not available, so in presenting their results they allow this K-factor to vary by a factor of 2 around that calculated for the SM Higgs.

I find this measurement interesting as an independent way to probe the Higgs width. In principle it is possible to increase the Higgs width to > 22 MeV and have it consistent with all measurements, but it involves scaling up SM couplings while adding a new decay mode to keep the production×decay rate fixed. The global Higgs fit means that the upper bound on the Higgs branching to unobserved (not necessarily invisible) decays is < 21% for an otherwise SM Higgs.

An arXiv preprint has shown that a one-parameter generic dissipative dark matter model with supernovae heat source can explain the 'wiggles' in rotation curve data. Assuming that DM cooling is balanced by this heating mechanism and the DM density is in a stable state, it is shown that the density of DM is related to the supernovae formation rate in the disk. If the SN rate is related to the gas density via a Kennicutt-Schmidt law, then the DM density is connected to the baryonic gas density in the disk: $$\rho(r,\theta)=\tilde\lambda \int d\tilde\phi \int d\tilde r \tilde r \frac{[\Sigma_{gas}(\tilde r)]^N}{4\pi[r^2+\tilde r^2-2 r \tilde r \sin\theta\cos\tilde\phi]},$$where $N\approx 2$, and $\tilde\lambda$ is an appropriately averaged quantity which depends on the cross section, supernovae dark photon energy spectrum etc.

You can now go out and apply this equation to gas density measurements and predict the rotation curve up to the constant $\tilde\lambda$. This was done for the spiral galaxy NGC1560 (below):

There are more examples in the paper, fitting to dwarf galaxies from the LITTLE THINGS survey released last month. Reasonably good fits are obtained, especially considering it is only a one-parameter model; the best fit values of $\tilde\lambda$ vary only within a factor of two.

The PICO-2L C3F8 Bubble Chamber in SNOLAB have reported the most sensitive direct detection constraints on WIMP-proton spin-dependent scattering to date. Below is a plot which compares various limits.

The limits which appear to "beat" PICO-2L come along with some assumptions: IceCube, ANTARES, and SuperK are neutrino telescopes looking for annihilating dark matter in the sun; CMS/ATLAS search for mono-X signatures and assume an effective field theory (valid if the mediator mass is $\gg\sqrt{\hat{s}}$). In the conclusion they claim "These limits represent... the first time supersymmetric parameter space has been probed by direct detection in the SD-proton channel." Can't help but doubt this...

The Neutrino Telescopes conference (NeuTel XVI) is well under way and is keeping a blog here which contains summaries of talks and posters. I really like this method for gathering in one place succinctly and accessibly the important information; of course one can still access the full talks to find out more.

Here is one interesting post from Francesco Iachello. He claims that quoted neutrinoless double beta decay bounds are too strong by a factor 2.5--6 due to the overestimate of a nuclear matrix element factor. This would mean that the inverted hierarchy region still allowed by Planck cannot be probed by $0\nu2\beta$ experiments in near future.

There is an interview with Jamie Bock of the BICEP2 experiment at Sean Carroll's blog. Here is an excerpt on the decision to release results when they did: The question really is, should we have waited until better data were available on galactic dust? Personally, I think we did the right thing. The field needed to be able to react to our data and test the results independently, as we did in our collaboration with Planck. This process hasn’t ended; it will continue with new data. Also, the searches for inflationary gravitational waves are influenced by these findings, and it is clear that all of the experiments in the field need to focus more resources on measuring the galaxy.

Twenty years ago on Tuesday was the discovery of the top quark at Tevatron. The papers of CDF and D0 were released together and can be read for free at PRL Milestones. Top at Twenty at Fermilab is celebrating this milestone from 9-10 April, in the traditional way of particle physicists: talks! Reviews of fundamental measurements of the top quark, measurements of top quark production and decay, theoretical talks on how the top quark fits into the Standard Model and its potential extensions, etc...

I don't know how significant this tweet from today is, but perhaps something to be aware of... [Edit: never mind, must have been minor (see second tweet)]

A paper published yesterday in Science has measured the D/H enrichment of atmospheric water on Mars. They found a D/H value enriched by a factor of about 7 relative to Earth’s ocean, which indicates that Mars has lost most of its water to space -- about six times the amount presently locked up in the Martian ice caps. This implies that Mars once had an ocean covering 20% of the planet's surface, up to one mile deep, and was wet for >1.5 billion years. Plenty of time for life! There's a 4 minute video about it from NASA Goddard, or an article at the guardian if you prefer.

Another paper in Science reports the observation of a gravitationally lensed supernova forming an Einstein cross. Because of the different travel times for light rays taking different paths around the lensing cluster, it is suspected that the supernova explosion would have appeared before (1964 and 1995) in another part of the sky, and will appear again elsewhere some time before 2020. There's an NY Times article here with a video.

Here is Rolf Heuer on the European Commission's decision to divert €2.7 billion (4%) of the EU’s science funding programme, Horizon 2020, to alternative investments.

Not sure what to think of the following tweet from Murdoch, but with The Australian's recent flip-flopping on Abbott who knows...

Ok, call it climate change. Can we have honest debate, and if true how bad and how much fault of humans?
— Rupert Murdoch (@rupertmurdoch) February 27, 2015

If you aren't already following Sabine Hossenfelder blogging at BackReaction, you should. Two more interesting pieces from her this week: 1) Are pop star scientists bad for science? 2) Can we prove the quantization of gravity with the Casimir effect? Probably not.

If you actually calculate it you find it to be $\approx 775$ GeV. Not so miraculous. Change the $\pi$ to a $\frac12$ and he does a lot better, but since we know that the Higgs mass is just a conspiracy of gravity, QCD, and the weak force, maybe he should have guessed $$ \approx \pi\left(\frac{m_p}{2m_W}\right)^8\sqrt{\frac{hc}{G}}, $$ at tree level of course.

About Me

Jackson Clarke, PhD candidate in phenomenological particle physics at CoEPP, University of Melbourne. Collider phenomenology, neutrino masses, and some naturalness. Science enthusiast, among many other things. Blogging accordingly.

Views are my own. Content very definitely skewed by my own leanings and by papers getting attention. So it goes.